Myopathology of myotonic dystrophy

Summary A histometric analysis of 16 biopsies taken from patients with myotonic dystrophy predominantly revealed type 1 atrophy, type 1 and 2 hypertrophy and type 1 predominance. These changes do not occur in all biopsies and are not pathognostic.Under the electron microscope otherwise apparently normal muscle fibres often demonstrated a single change: a swelling and proliferation of the sarcoreticulotubular system in the I-band region. Morphometry of randomly selected fibres from 15 biopsies revealed a significant increase in the sarcotubular membrane profile concentration in longitudinally and in transversally oriented sections. In addition, a shifting and disappearance of thin filaments in the I-band was noted to be an early sign. Pathological changes in the plasma membrane such as excessive foldings, expulsion of vesicular material and periodic densities were also seen. The morphometric analysis did not reveal any significant change in the mitochondrial fraction and mean mitochondrial size. All the ultrastructural changes of skeletal muscle decribed in myotonic dystrophy are unspecific. Even the alterations of the plasma membrane and the sarcoreticulotubular system described here may not be a primary correlate to myotonia, but represent secondary alterations or may belong to the dystrophic changes. Spontaneous human and experimental myotonia may exist without such alterations.Some biopsies demonstrated light and electron microscopic findings which may indicate a neurogenic process. However, it appears more probable that both the nerve and the muscle may be independently affected by the pleiotropic action of the responsible gene.